The present invention relates to a device for separating floating layers, e.g., mud, foam and the like, from a liquid residing in a liquid basin, e.g., a settling basin, and carrying the floating layer, wherein the device includes a transport device which moves the floating layer to a transport discharge region.
Devices of the aforedescribed type are used wherever a medium, such as mud or foam, that floats on a liquid is to be separated from the liquid. Most frequently separated or removed is floating mud produced in final sedimentation tanks or mass separation facilities. Floating foam can collect in the form of a floating layer and reach a height on the surface of the liquid of close to 10 cm. With layers of this thickness, it is particularly important that the discharge overflow edge remains very precisely adjusted relative to the liquid level, so that the mixture consisting of the floating layer and the liquid can be withdrawn without noticeably stirring the liquid remaining in the final sedimentation tank. Otherwise, the floating layer may not properly separate.
Processes applied hitherto in the field of sewage treatment to solve this problem have failed and produced separation of poor quality. Moreover, in most situations the separation process has to be supervised so that the facilities operate properly. This results in relatively high costs and a rather low efficiency.
For example, WO-A-92/01516 describes a device for separating floating mud and wastewater in a closed container which has a lower inlet for the wastewater and an outlet with an overflow pipe which can be used to adjust the level in the wastewater container. A worm conveyor is arranged in the upper region of the container. One end of the worm conveyor is connected through a drive shaft to a stationary drive unit, while the other end is connected to a discharge cone for carrying the mud away with an upwardly slope. Other embodiments are depicted wherein the worm conveyor for transporting the mud is sloped upwardly. The changes in the liquid level are adjusted via the overflow pipe, while the worm conveyor remains in a fixed position with respect to the housing. In practice, it is difficult to maintain a constant liquid level wherein the overflow pipe is at a constant height due to the sluggish response of the system. Even if the contact depth of the worm conveyor changes only during a brief period, the floating layer and the liquid may be inadvertently stirred or mixed. Many applications operate advantageously with a constant feed ratio of mud to liquid which can be adjusted from turbid to diluted. This is difficult to implement if the worm conveyers are installed in a fixed position and the liquid level changes constantly. Moreover, the device becomes more susceptible to malfunction.
WO-A-95/23334 describes a device for separating floating foam wherein a cylindrical frame on which honeycomb-shaped lamellae are disposed, is moved inside a basin. The lamellae subdivide the contents of the basin into separate liquid columns which are sealed off from each other. In this way, the individual lamella channels are not adversely affected by the turbulence produced by the moving device. The liquid level is located slightly above the upper edge of the lamella arrangement, so that the floating foam is entrained when the lamellae rotate and is thereby transported into a mud collection device. The mud collection device has a blade and a worm conveyor which moves the stripped-off floating mass in an axial direction to an overflow edge and from there to a collection line. Because the worm conveyor is stationary with respect to the basin, the quality of the separation again strongly depends on the liquid level in the basin. It is difficult to prevent clarified water and floating foam from intermixing inadvertently.
U.S. Pat. No. 3,709,357 shows an arrangement for removing a floating oil layer from a water surface. A flexible worm conveyor which is designed to be lighter than water and thus floats on water, extends between two boats. The drive motor located on one of the boats drives the flexible worm conveyor rotates and thereby moves the oil mass to a hose entrance port. The floating worm conveyor is constructed from twisted cable strands which retain the conveyor blades around a center strand in their respective position. However, removing an oil layer floating on the water surface from seawater can hardly be compared with the tasks encountered in sewage treatment applications. First, a settling basin presents spatial conditions which are entirely different from those of the open sea, and secondly, a freely expanding oil layer behaves very differently from the floating layer masses which may be present in a final sedimentation tank and can have a height of up to 10 cm. The cable assembly illustrated in this patent cannot be applied to the transport of floating mud, since the fiber fraction and the solid fraction of the floating layers would immediately adhere to the large and rough surface of the cable assembly, thereby clogging the cable assembly and preventing further transport. Moreover, mud would also adhere to and accumulate at the cable assembly in the radial direction, thereby impeding the axial transport effect of the screws. The arrangement illustrated in U.S. Pat. No. 3,709,357 for transporting floating substances is therefore directed exclusively to a liquid which--as shown in FIG. 2 of the referenced patent--is pumped through a hose section into a discharge pipe. It is not feasible to apply such removal process to a final sedimentation tank.
U.S. Pat. No. 4,196,087 discloses a floating system for removing liquid surface layers. A central collection container for the liquid is surrounded with a float-like arrangement which drives the entire unit. Lateral inlets are provided for receiving floating liquids and/or contaminants. Worm conveyers surround the collection float, ext ending radially away therefrom. The worm conveyers consists of buoyant cylinders having two helical surfaces arranged thereon, with one end of each cylinder coupled to a drive unit producing a cylinder rotation. The worm conveyers can be used to transport the floating liquids to the collection inlets, from where the liquids are transported by another system consisting of counter-rotating helical surfaces and across a ramp into the collection tank. The worm conveyers are driven from a floating location. They are, however, mostly located beneath the liquid surface, making them unsuitable for transporting a floating layer mass without stirring the liquid. This "offshore" system does not form a combined catchment and transport system, but merely a transport system with upwardly sloping discharge elements.
U.S. Pat. No. 3,447,683 discloses a device for separating contaminants floating on a liquid surface, with the device consisting of two communicating tanks. The floating layer is driven by a worm conveyor from a larger tank into the smaller tank. A dense floating layer, which improves the separation process due to the differences in density and the connection via the communicating tanks, is thereby produced in the smaller tank. The separation of the floating layer is relatively imprecise due to the stationary arrangement of the worm conveyor.
U.S. Pat. No. 4,784,764 discloses an arrangement for flocculating substances from a suspension. The flocculated substances are drawn off the liquid surface of a basin with the help of a conveyor system and discharged through a discharge pipe. The conveyor system is formed by a worm conveyor (FIG. 4) located in a semi-cylindrical channel. The overflow channel is located above the level of the flocculated layer. The floating layer is lifted by a paddle wheel above the overflow channel, thereby mixing the floating layer mass and the liquid, which lowers the quality of the separation.
Also known are tiltable channels wherein the height of the overflow edge can be adjusted in such a way that the floating mud can be removed by lowing the overflow edge. The overflow edge is here lowered uniformly across its entire length which disadvantageously causes the floating mud particles to be mixed non-uniformly with the underlying liquid when reaching the discharge channel. This results in a poor separation between the floating layer and the underlying liquid. Moreover, this device cannot be adapted at all, or only in a limited way, to strongly varying liquid levels.
Other known arrangements use revolving chain drives and laterally disposed scum boards to separate the floating mud, wherein the floating layer mass collects in front of the scum boards. The floating layer mass is transported sideways to a pump container by paddles mounted on the chain drives and is drawn out of the pump container. The moving drive elements may experience excessive wear and corrosion which makes this arrangement expensive to manufacture and to maintain, reducing its overall cost-effectiveness. Moreover, with this arrangement, the inflow is disadvantageously restricted to only one side.
Other arrangements for separating floating mud used with circular basins include a first floating mud barrier, with a second floating mud barrier positioned in front of the first floating mud barrier. The second floating mud barrier is periodically moved towards the first floating mud barrier, so that the space between the first and the second mud barriers forms a channel. In the position where the first and second mud barriers form the channel, individual paddles drive the floating mud to the transport discharge region. These arrangements have also the disadvantage of high manufacturing and maintenance costs, and inflow is restricted to only one side.
Also known are rotatably driven discharge ducts with slots and paddles which allow floating mud to enter the slots depending on the rotational motion. These arrangements, however, are disadvantageously much less precise for separating the floating matter and the harmful clarified water fraction because the distribution of the floating matter across the entire width of the inlet is not properly taken into account.
Finally, another arrangement uses discharge channels for the floating mud, which have sloping inlet planes before the inlet edges and crank mechanisms with strip-like brushes to swipe the floating layer into the channel. Disadvantageously, these arrangements operate only over short distances and over a limited range of liquid levels, and inflow is restricted to one side.
All of the aforedescribed arrangements have the additional disadvantage that floating matter can accumulate heavily in certain areas and can permanently dry in areas that are not adequately wetted. Any cleaning required reduces the up-time of the machine.